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• What is the Science Citation Index®?
• What is the Social Sciences Citation Index®?
• What is the Arts & Humanities Citation Index®?
• What is the Web of Science®?
• What are the Journal Citation Reports®?
• What is an impact factor?
• What should be considered when evaluating journal impact factors?
• How does a journal get an impact factor?

Impact factors

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Thomson Reuters, citation indexes, Journal Citation Reports®, and impact factors.

Thomson Reuters was formed in 2008 with Thomson's acquisition of Reuters.
The most relevant part is Thomson Reuters Scientific, which runs various abstracting and indexing databases, as well as publishing impact factors. It is often still referred to as ISI (Institute of Scientific Information), a company started in the 1950s by Eugene Garfield. ISI created and produced the Science Citation Index®, the Social Sciences Citation Index®, the Arts & Humanities Citation Index® and the Journal Citation Reports®. In 1992 ISI was acquired by the Thomson Corporation and for a while was known as Thomson ISI and then later as Thomson Scientific. The division is now officially known as Thomson Reuters Scientific.

What is the Science Citation Index®?

The Science Citation Index® (SCI®) is a literature search tool available on DVD, providing access to bibliographic information, abstracts, and reference lists from over 3,700 scientific, technical, and medical journals.

The Science Citation Index Expanded^TM, available online through the Web of Science®, covers all the journals in Science Citation Index® along with over 4,000 additional titles.

What is the Social Sciences Citation Index®?

The Social Sciences Citation Index® (SSCI®) is a literature search tool available online via the Web of Science®.

It provides access to bibliographic information, abstracts, and reference lists from over 2,600 journals in the social sciences and from selected articles from approximately 3,300 scientific, technical, and medical journals. There is no expanded version of the Social Sciences Citation Index®.

What is the Arts & Humanities Citation Index®?

The Arts & Humanities Citation Index® (A&HCI®) is a literature search tool available online via the Web of Science®.

It provides access to bibliographic information, abstracts, and reference lists from over 1,530 arts and humanities journals and from selected articles from approximately 7,000 journals in the sciences and social sciences. There is no expanded version of the Arts & Humanities Citation Index®.

What is the Web of Science®?

The Web of Science® is Thomson Reuters' web interface, which allows searching across the Science Citation Index Expanded^TM, the Social Sciences Citation Index®; and the Arts & Humanities Citation Index®, covering a total of 11,100 journals.

The Web of Science® also allows searching across additional databases owned or licensed by Thomson, such as Index Chemicus, BIOSIS Previews and MEDLINE.

What are the Journal Citation Reports®?

The Journal Citation Reports® (JCR®) are annual reports produced by Thomson Reuters which allow the evaluation and comparison of journals based on citation data. The citation data in the Journal Citation Reports® is collected over one year and published the next June.

For example, the 2009 Journal Citation Reports® are based on citation data from 2009 but were published in June 2010. The Journal Citation Reports® come in two editions, the Science Edition, covering the journals included in the Science Citation Index-Expanded^TM, and the Social Science Edition, covering the journals in the Social Sciences Citation Index®. Some journals are listed in both editions. There is no Arts & Humanities edition.

It should be noted that unless a journal is selected for coverage in its first volume, it will take two to three years after coverage begins in the Science Citation Index-Expanded^TM or Social Sciences Citation Index® for a journal to be listed in the Journal Citation Reports®. For example, a journal selected for coverage in 2010 should be listed for the first time in the 2012 Journal Citation Reports®.

The metrics published yearly in the Journal Citation Reports® include the journal impact factors.

What is an impact factor?

Impact factors give the average number of citations received by articles in a particular journal; essentially, the average number of times that articles in a particular journal are referenced by other articles.

The impact factors published annually in the Journal Citation Reports® are defined as follows:

Number of citations (references) received in the impact factor year to articles published in the two previous years, divided by the number of articles published in these two years.

Therefore, the 2009 JCR impact factors (released in 2010) were calculated as follows:

For example, the 2009 Impact factor for Biofouling was calculated as follows:

Citations received in 2009 to articles published in Biofouling in 2008 = 207
Citations received in 2009 to articles published in Biofouling in 2007 = 208
Total citations received in 2009 to articles published in 2007 and 2008 = 415

Number of articles published in Biofouling in 2007 = 40
Number of articles published in Biofouling in 2008 = 54
Total number of articles published in 2007 and 2008 = 94

What should be considered when evaluating journal impact factors?

The following are the main things that need to be considered when comparing or evaluating journal impact factors:

Subject variation
The average number of citations received by articles during the two years after publication varies considerably across different subject fields. This leads to very different ranges of impact factors in different subject areas. For example, the top journal in cell biology has an impact factor of more than 40. In law, however, the top journal has an impact factor of less than 5. This doesn't mean that cell biology journals are 'better' than law journals; it is simply a reflection of different referencing patterns and behavior in these fields.

The Journal Citation Reports® take this variation into account by dividing the journals into subject categories. It is only within these categories that impact factors should be compared, and a journal's relative standing in a category is generally more important than the actual value of its impact factor. There are over 220 subject categories in the two editions of the Journal Citation Reports®.

You may see the following type of information on a journal homepage:

This means that the journal has an impact factor of 4.415 and this is the second highest impact factor out of the 88 journals listed in the "Marine & Fresh Water Biology" category and also 19th out of the 150 journals listed in the "Biotechnology & Applied Microbiology" category.

Basic versus applied research
Applied journals are more likely to reference related basic research journals than other applied journals. There is no comparable flow of citations back from the basic research journals. Thus, basic research journals tend to receive more citations than related applied journals and, therefore, have higher impact factors. Practice-based and educational journals often have particularly low impact factors compared to the basic research journals in their fields. However, these journals fulfill a necessary role within their community.

Article types
Review articles are generally cited more often than primary research articles. This is because authors will often cite one review article rather than the many primary research articles it is based on. As such, review journals, or journals that publish a significant amount of review content alongside their primary content, usually have higher impact factors than other journals in their field. For example, the average¹ 2003 impact factor for review journals published by Taylor & Francis, Routledge, Psychology Press, and Informa Healthcare was 3.5 times higher than the average for our primary research journals.

Although the JCR® lists journals from different subject areas separately to take account of subject variation in impact factors, it does not list review journals separately from primary research journals. Therefore review journals are often ranked amongst the highest journals in their fields. For instance, three of the top five journals in the 2009 "Chemistry Multidisciplinary" category were review journals and the other three journals published between 6% and 60% review articles.

Journal size and Impact factor variability
A journal's impact factor can change a great deal from year to year and the smaller the journal is, the more variable its impact factor is likely to be. This is because small changes in the absolute number of citations received have a much larger effect on the average number of citations when the denominator (number of articles) is small.

For example, we can consider the effect of publishing an article which receives five citations, on two theoretical journals with an impact factor of 1.000, one publishing 25 articles a year and the other 100 articles a year. In the small journal this will improve the impact factor by 0.100 (5/(2x25), which is a 10% improvement. In the large journal, however, this would only lead to an improvement of 0.025 (5/(2x100) which is only a 2.5% improvement.

How does a journal get an impact factor?

Journals need to be selected for coverage in either the Science Citation Index-Expanded^TM or the Social Sciences Citation Index® before they will be listed in the Journal Citation Reports® and given an impact factor.

Thomson Reuters considers many factors when evaluating whether to cover a journal in one of its citation indexes. These include the level of citation activity to the journal from the titles that are already indexed, basic publishing standards such as getting issues published on schedule, and the international relevance of the journal. However, the subject coverage of the journal is also an important factor and journals in well-covered or low-priority subjects may struggle to get selected.

As mentioned above, it should be noted that even if a journal is selected for coverage in the Science Citation Index-Expanded^TM or Social Sciences Citation Index®, it will be two to three years after coverage begins before the journal is listed in the Journal Citation Reports®, unless the journal is selected for coverage in its first volume. For example, a journal selected for coverage in 2010 should be listed for the first time in the 2012 Journal Citation Reports®.

More Information
For more information on these databases and metrics, please see Thomson Reuters' product pages and free essays.

¹ This refers to a weighted average which takes journal size into account.